The field of the invention relates to idle speed control systems for motor vehicles having fuel vapor recovery systems coupled between the fuel system and engine air/fuel intake.
Feedback idle speed control systems are known which control a bypass throttling device, connected in parallel with the primary engine throttle, in response to a difference between desired idling speed and actual idling speed.
The inventor herein has recognized at least one problem with such idle speed control systems. When the fuel vapor recovery system is purged into the engine air/fuel intake during engine idle control, the purged flow may be greater than the airflow required for desired engine idling. Accurate control of engine idling speed may therefore be unachievable under all engine operating conditions. For example, the engine idle may surge even though the bypass throttling device is fully throttled.